How the World Trade Center Slurry Wall Works

When plans were laid to create a September 11 museum as part of the reconstruction of Ground Zero, it seemed obvious to many that at least part of the original slurry wall had to be included in the memorial. As Stefan Pryor, president of the Lower Manhattan Development Corporation, explained to The New York Times in 2005: "Now that the slurry wall has been laid bare and infused with meaning, it's our obligation to preserve it and ensure that all who come to the site have the opportunity to view it and pay tribute at it" [source: Dunlap].

The museum's construction budget only allowed the builders to spend $11 million to include a small section of the original slurry wall, consisting of three 20-foot-wide (6-meter-wide) panels. According to a 2008 New York Times article, the process of protecting the slurry wall from damage during the reconstruction project was elaborate. All the digging around the wall was done manually, for extra care. A new concrete liner was poured in front of the original slurry wall to strengthen it from caving in or leaking, and high-strength steel cables were put in place to give the support that the towers' basement once provided. Additionally, to protect the slurry wall's surface from deterioration, it was covered with a protective coating of liquefied concrete known as shotcrete [source: Dunlap].

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But museum officials hope the end result will be a moving experience for visitors. Museum president Joseph C. Daniels told The New York Times in 2008 that he envisioned the slurry wall becoming as iconic as Jerusalem's Western Wall, also known as the Wailing Wall, the last remaining section of the city's Second Temple and a sacred spot for members of the Jewish faith. "The idea of being able to get that connection, which will link you to the past, is important," he said [source: Dunlap].

The Geology of Ground Zero

The World Trade Center was built just west of where the first Dutch explorers landed in 1614, on a landfill that had been amassed over several centuries of Manhattan's evolution. The composition of the landfill varied from east to west. At the east end, it was mostly sand, silt and crushed rock, atop a surface of bedrock about 65 to 80 feet (19.8 to 24.4 meters) below. To the west, the mix changed to soft river mud, atop a layer of sand, silt and rock, with bedrock at depths of 55 to 75 feet (16.7 to 22.8 meters). Groundwater flowed through the landfill within several feet of the surface. Additionally, the landfill contained bits and pieces of demolition debris from old buildings, abandoned vessels and cargo, garbage, and old utility lines [source: Tamaro].